如何在不输入数字的情况下定义数学常数？

Question

I need to use the square root of two, 1.414..., in a loop.

Obviously, I don't want to call the function Math.Sqrt(2) all the time. Sure, it's a single instruction on modern processors, and the JIT or compiler will probably figure out what's going on, but I want my code to be clear, readable, and fast.

Because I want my code to be clear and readable, I'd prefer to define this constant by calling sqrt2 = Math.Sqrt(2) instead of typing in the magic number sqrt2 = 1.4142135623731d .

Finally, because this value is a constant, I want to declare it with the const keyword. But when I write:

const double sqrt2 = Math.Sqrt(2);
// const double sqrt2 = 1.4142135623731d;

the compiler complains:

Error 1 The expression being assigned to 'sqrt2' must be constant

How do I best define this value?

Answer 1

Math.Sqrt is a method call, so that cannot be assigned to a compile-time constant.
You can only assign it to a run-time constant :

static readonly double sqrt2 = Math.Sqrt(2);

Answer 2

You can try to use static readonly instead of const .

static readonly double sqrt2 = Math.Sqrt(2);

Do understand that when you declare a constant then it has to be compile time constants.

The C# spec says:

A constant expression is an expression that can be fully evaluated at compile time.

Answer 3

I see no reason that you shouldn't initialise it as a constant.

In my opinion, if you use a named constant for a number, then that number is not a magic number.

You could create a MathematicalConstants class to hold that, and other values:

public static class MathematicalConstants
{
    public const double SQRT2 = 1.4142135623731d;
    public const double SQRT3 = 1.7320508075689d;
}

Then if you're using C#6 or later, you can put

using static MathematicalConstants;

at the start of your code module, and you wouldn't need to refer to it as MathematicalConstants.SQRT2 . You could just use SQRT2 in your code:

const double sin45 = 1/SQRT2;
const double tan30 = 1/SQRT3;

Incidentally, there's a precedent for putting constants in static classes. The Math class has PI and E as constants, and they are initialized directly in the source code.

Answer 4

In C# constant initializer must be compile const because they are embedded as literals in place of usage during compilation. This means that every usage of a member marked as const will be replaced with its value during compilation, while readonly members will be resolved at run-time. You can use static readonly instead.